<!DOCTYPE HTML>
<html>
<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
    <title>无标题文档</title>
</head>
<body>
<input type="button" value="换背景吧"/>
<input type="button" value="停"/>
<div id="back"></div>
<script>
    var aBtn = document.getElementsByTagName('input');
    var arrUrl = ['img/1.jpg', 'img/2.jpg', 'img/3.jpg', 'img/4.jpg'];
    var num = 0;
    var timer = null;
    var oBody = document.getElementById('back');
    aBtn[0].onclick = function () {
        clearInterval(timer);			// null、未定义
        timer = setInterval(function () {
            oBody.style.background = 'url(' + arrUrl[num] + ')';
            num++;
            num %= arrUrl.length;
        }, 1000);
    };
    aBtn[1].onclick = function () {
        clearInterval(timer);
    };
</script>

</body>
</html>
